# Kinetic energy

1. Jan 13, 2008

### mortho

1. The problem statement, all variables and given/known data

A 7.0 g coin moving to the right at 22.0 cm/s makes an elastic head-on collision with a 19.0 g coin that is initially at rest. After the collision, the 7.0 g coin moves to the left at 12.5 cm/s.
Find the amount of kinetic energy transferred to the 19.0 g coin.

2. Relevant equations

m1v1+m2v2
1/2mv2

3. The attempt at a solution

so i found the final velocity of the 19.0 g coin and it was 12.9 cm/s to the right and i used the kinetic energy formula and set everything equal to eachother and then subtracted and got 660 J but it's wrong. This was just the difference so i'm guessing thats why its wrong. But i don't know how to find just the energy transfered. Is that the same and difference? Thanks

2. Jan 13, 2008

### mortho

Does anyone know if i'm just supposed to subtract or what? thanks :)

3. Jan 13, 2008

### Feldoh

Since the question asks about energy transferred, I take it that it's an elastic collision in which case energy is conserved (Ki=Kf)

The 19g coin initially has no kinetic energy so whatever it's kinetic energy is after the collision is the amount transferred.

Other then that watch your units.

4. Jan 13, 2008

### mortho

so is it going to be only the 19.0 g coin itself or 1/2mv2+1/2mv2 of the 7 g and 19 g coin? in which case for both combined is 2128 J or by itself 1581 J

5. Jan 14, 2008

### Feldoh

Just the 19g coin, the question only asks for the energy from the 19g coin. J=Nm are you sure your units are right? It looks a bit high.

6. Jan 14, 2008

### mortho

Yes my assignment is already due but i figured what i had done wrong. My units weren't correct. i was supposed to convert the cm/s but the problem hadn't stated that so i might have a chance. Thanks for everyone's help! :)

7. Jan 14, 2008

### kbaumen

The problem doesn't have to state that. m/s is the standard unit in SI system, so if you use any other unit for speed in these formulas, the result will be incorrect.